Arc welding plants are known which are designed for the automatic or semi-automatic welding of metal parts by the continuous deposition of weld beads in a bevel groove formed at the junction of the two parts.
In certain cases, the welding operation must be controlled and monitored from a distance in order to avoid the presence of an operator in the zone where the parts to be joined are situated.
This is in particular the case in the welding of parts of large dimensions in the primary circuit of a pressurized water nuclear reactor.
With a view to lengthening the life of a nuclear power station, it has recently been proposed to use new steam generators to replace used steam generators in a pressurized water nuclear reactor in which the bank of tubes has become defective in the course of long utilization.
For the purpose of carrying out this operation of replacing the whole of a steam generator, it is in particular necessary to join, by welding, the primary water inlet and outlet connections of the steam generator to the corresponding primary circuit pipes held ready in the reactor building. This operation can be carried out by semi-automatic welding, the operation being controlled from a control station situated at a certain distance from the welding zone. An operator must permanently monitor the welding operation with the aid of a visual display, on a television screen, of the zone in which the weld bead is formed.
In this case, it is necessary to provide a very clear image of the welding zone, because with the aid of the image on the screen the operator must evaluate any action that must be taken to achieve the deposition of a perfect weld bead. It is in fact necessary to obtain a deposit of weld metal which is free from defects.
The arc welding plants used for welding pipes of large dimensions are of the orbital type and incorporate a motorized support member mounted for movement on a circular rail fixed on one of the pipes being joined. On this support member is mounted a TIG welding head incorporating an electrode permitting the refusion of filler wire continuously fed to the zone of the arc struck between the electrode and the junction zone of the parts. The welding head is mounted for oscillation in the transverse direction in relation to the orbital movement of the welding machine. A sweep is thus made over the width of the weld bead.
Televisual observation of the welding zone is therefore complicated by the existence of a very bright spot or zone, corresponding to the arc moving over the width of the weld bead, at the center of the welding zone. The weld bead on which the operator must concentrate his attention may be poorly visible because of the very high luminous intensity of the arc. It is possible to use an opaque filter, but the parts of the welding zone which are not directly subjected to the light of the arc will then be poorly visible, even if floodlights are used to illuminate the welding zone. To achieve effective monitoring of the welding in progress, the operator must be provided with an image of the entire bevel groove which is sufficiently clear, in particular, for verification of the state of the bead just deposited and its position in relation to the other beads deposited previously, and also for monitoring the progress of the welding.